1. Field of the Invention
The invention, in general, relates to films or foils and products made from, or incorporating, foils or coatings and, more particularly, to such products made from aqueous solutions of starch or amylose derivatives of the kind useful as fat impervious coatings of otherwise pervious or wettable packages made of paper, card board and the like.
2. The Prior Art
Films, foils and foil products, hereinafter sometimes collectively referred to as foils, are produced in huge quantities. Since a considerable quantity of such foils is intended for single time use only and since, following such use, the foils are usually discarded, there is an increased demand for foils which can either be recycled or which are biodegradable. Recycling is, however, sometimes made difficult since such foils, because of their great variety, can often not be distinguished and sorted by laymen or consumers for subsequent recycling processes. Foils which are fully bio-degradable or compostable within a reasonable time would, of course, constitute an ideal alternative to conventional foils and would very likely eventually replace those conventional foils which are not biodegradable or compostable, such as polyvinyl chloride, polystyrene, polyethylene, etc., if they could be made to posses similar properties, especially in terms of transparency, mechanical strength and stability as well as unidirectional or bidirectional imperviousness, as the case may be, in respect of predetermined substances.
Heretofore, numerous experiments have been conducted in which starch was used as a base or raw material for producing bio-degradable films or foils. However, products made from pure starch have thus far not been able effectively to compete with conventional products made from synthetic polymers. Their low tear strength and extensibility as well as their lack of transparency and stability are drawbacks which have prevented use of those products on any significant scale.
To overcome the disadvantages and, more particularly, to improve the mechanical properties of starch-based foils, it has been proposed (see, e.g. German patent 1,745,680) to add to the starch various softeners such as sorbite, glycerine, polyvinyl alcohol and so forth. Another way of improving the mechanical properties of starch-based products of the kind here under consideration is to add polyolefin-based synthetic polymers. For Instance, WO 90/14388 discloses a biodegradable plastic formed from corn starch and low-density polyethylene. The improvements brought about by such products have not, however, been such as to result in serious alternatives to synthetic polymers.
Another attempt to avoid, or at least lessen, the mentioned disadvantages was made by using so-called special amyloses such as, for instance, chemically modified high amyloses. For example, certain hydroxyalkyl derivatives of amylose may be processed into thermoplastic articles or foils, films and membranes, or hydroxypropyl starches may be added to other polymers, with a view to attaining improvements in predetermined properties. Thus, U.S. Pat. No. 3,850,652 relates to a specific application and teaches a multi-purpose membranous material of high flexibility and mechanical strength formed at least in part of a hydroxypropyl starch. Whilst this multi-purpose membranous material may be immune from being absorbed by other materials in contact therewith or vice versa, it is neither transparent nor completely biodegradable.
As described in EP 0,417,828, hydroxypropyl starches have also been used to improve the biological degradation behavior of products made from unsaturated hydrocarbon polymer compounds which would otherwise be difficult to decompose biologically. However, the properties of such products have by no means been satisfactory. Whilst the products may be generally transparent, they nevertheless display some slight brownish tinge, and they are of insufficient mechanical strength. Furthermore, U.S. Pat. No. 3,778,392 describes a mixture of polymeric starch and hydroxypropyl starch for use in connection with high-quality papers. These products are not biodegradable and involve comparatively complex manufacturing processes.
As regards their utility, none of the known products derived from starch or amylose have developed into products which effectively compete with comparable synthetic polymer products. Moreover, additives and softeners added to the starch or amylose often prevent complete biodegradability of the final product.
It is an object of the invention to provide foils or products thereof made from starch or amylose derivatives without added synthetic materials.
Another object is to provide foils or products thereof made from starch or amylose derivatives without added softener.
Still another object is to provided foils and the like made from starch or amylose derivatives which are qualitatively significantly superior to known foils of this kind by providing improved physical and mechanical properties.
It is a particular object of the invention to provide foils and the like made from starch or amylose derivatives which are of superior tear strength and extensibility as well as high transparency.
It is also an object of the invention to provide foils and films made from starch or amylose derivatives which may be used as overhead projection slides or printable films.
Yet another object of the invention resides in the provision of foils and films which may be used as surface coatings or laminates of paper and cardboard.
Still further, it is an object of the invention to provide a compound made from starch or amylose derivatives which may be deposited on a surface by spraying, spreading or pouring to form a foil or film thereon.
It is a further object of the invention to provide a compound from starch or amylose derivatives suitable as a surface coating of paper and the like to improve the surface quality thereof.
It is yet another object of the invention to provide a compound derived from starch or amylose derivatives which is completely biodegradable.
Another important object of the invention is to provide a laminate consisting of paper coated by a compound derived from starch or amylose derivatives to provide a substantially fat impervious food wrapping material.
A still further particular object of the invention is to provide a laminate incorporating a layer of a starch or amylose derived substance to provide a high degree of imperviousness to fat or fatty substances.
Still another object of the invention resides in the provision of a material suitable for coating a support layer made of paper, cardboard or similar materials to render it substantially impervious to fat or fatty substances.
Other object will in part be obvious and will in part appear hereinafter.
In accordance with the invention, there is provided a compound derived from a starch or amylose derivative of low salt content and dissolved in ether, the derivative having a mean molecular weight of between about 105 and about 2xc3x97107 g/mol, a degree of substitution between about 0.1 and 0.5, a substantially uniform distribution of substituents between the C2/C3 and C6 within their glucose unit, a water content of less than 25% under normal climatic conditions, a polydispersity from about 2 to about 8, and a predominantly amorphous structure with a low crystalline proportion and a predominant particle size not greater than 400 nm.
In a further advantageous embodiment of the invention the basic material has an amylose content of not less than 60%.
Other advantageous embodiments will become apparent to those skilled in the art as the description unfolds.
It has surprisingly been found that foils and products made thereof possessing the above parameters and made from pure starch or amylose derivatives without any additives have excellent mechanical properties and a high degree of transparency. Due to the fact that the foils and products made from them do not incorporate any synthetic additives or softeners they are completely biodegradable.
Particularly advantageous results are attained if the basic material, i.e. the starch, is derived from the group of high amylose starches and if its amylose content is not less than 60%.
The inventive parameters of the derivative, viz. low salt content, mean molecular weight between about 105 and about 2xc3x97107 g/mol, polydispersity of about 2 to about 8, a degree of substitution of 0.1 to 0.5 and preferably from 0.1 to 0.3, substantially uniform distribution of substituents between the C2/C3 and C6 bond sites within the glucose unit and a low proportion of hard-to-dissolve components are obtained by a careful hydroxypropylation and desalination. Following the hydroxypropylation and desalination the derivative will be a colloidal, visco-elastic solution which may be further processed by known processes such as casting, spraying, spreading and the like into foils and products made thereof with a predominantly amorphous structure of low crystallinity and a mean particle size of 400 nm below the limits of visibility, the latter being an important requirement for the high degree of transparency of the foil or products made from it. The water content of the foil significantly influences its mechanical properties. Under normal climatic conditions, it is below 25 percent by weight. The foils are of high transparency and, compared to conventional foils and foil products made from starch or amylose derivatives, are of significantly improved stability and extensibility. Owing to the relatively constant water content of the foils or foil products, these properties remained substantially unchanged even after long storage periods at a relative humidity between about 30% and about 70% and at a temperature of about 23xc2x0 C.
The foils and foil products in accordance with the invention have an internal as well as an external plasticization effect which may on the one hand be ascribed to enlarged spaces between individual macromolecule resulting from the chemical addition of sterically large side groups which lead to increased movability of the polymer chains. On the other hand, it may be ascribed to low molecular groups into which the hydroxypropylated amylose may easily penetrate or physically bonded, thus acting as external softeners. In the present case, the low molecular groups are, in fact, water acting as a softener.
In this connection, the starch concentration or overlapping density of starch molecules is of great significance as it determines the way and progress in which the water molecules penetrate or are physically bonded. Thus, the mechanical properties are influenced as well.
Accordingly, the foils or products made thereof in accordance with the invention require no softeners or additives. This results in a complete biodegradability or compostability of the foils and products. It also prevents the foils or foil products from becoming brittle. Furthermore, materials in contact with the foil or foil product cannot be detrimentally affected by migration of a softener. The mechanical stability of the foils in the range of relative humidity from about 30 to about 70% at a temperature of about 23xc2x0 C. is excellent.
The predominantly amorphous structure and a predominant particle size below 400 nm impart excellent optical properties to the foil or foil product. The degree of transmission at light in the range of 560 nm was up to about 93%. The foils are, therefore, especially well suited as overhead projection slides. They may also be used as data carriers for they may be written or printed upon. They may also be used as copy surfaces. The mentioned properties make the foils particularly well suited for ink jet printing. To this end, a surface of a support material may be enhanced or refined by applying to it a coating of an aqueous solution of starch or amylose derivative. Suitable carrier materials are cellulose or other natural fibers. Of course, synthetic support materials may be used as well; but it should be understood that it would probably interfere with the biodegradability of the foil product.
It is also an aspect of the invention to utilize an appropriately constituted starch or amylose derivative in the manufacture of paper and cardboard in order to improve not only their mechanical and optical properties but also their surface characteristics.
Preferably, the foils or film coatings in accordance with the invention have a thickness between about 20 and 200 xcexcm; and their preferred density lies in the range of 1.3 g/cm3.
The properties of the foils and foil products in accordance with the invention may be summarized as follows:
They have a tensile strength  greater than 40 Mpa (megapascal) at normal climatic conditions and thus they clearly exceed conventional foils made from starch with a high amylose content as well as 81/15 copolymer lactide/caprolactone (22.0 MPa) and soft polyvinyl chloride (16-34 MPa);
Their elongation at break is up to 60 Mpa and they are thus superior to low density polyethylene (longitudinal 18-32 MPa; transversal 16-25 MPa) as well as medium and high density polyethylene (longitudinal 20-40 MPA; transversa 15-30 MPA);
Their E module is between about 10 to about 40 MPA;
They possess high transparency ( greater than 80% at light of a wavelength of about 560 nm);
They have a high blocking efficiency as regards gases, especially nitrogen and oxygen;
They may be stored at relative humidity levels between about 30% and about 70% at a temperature in the range of about 23xc2x0 C.;
They are stable against discolorations at temperatures up to 170xc2x0 C. for 15 minutes;
They may be welded, deep drawn, written, printed or copied upon;
They have excellent adhesive and bonding properties when applied from an aqueous solution onto a carrier material such as, for instance, cellulose or natural fibers, and they markedly improve the surface characteristics thereof; and
In preferred embodiments they are free of any softeners or synthetic additives and completely biodegradable as defined by E-DIN 53 739 in view of the fact that they are biological polymers.
Given their mentioned properties, the foils and foil products in accordance with the invention, for specific applications, constitute genuine alternatives to foils or foil products made from synthetic polymers such as polyvinyl chloride, polyethylene or polystyrene. If the foils and foil products are used in connection with other materials, their biodegradability does, of course, also depend upon those other materials.
In order to impart specific properties to the foils or foil products predetermined materials or material systems may be added which are not biodegradable. In such circumstances the complete biodegradability relates only to the foils and foil products in accordance with the invention.
It has surprisingly been found that, if properly constituted, starch or amylose derivatives in accordance with the invention have excellent properties in respect of fat imperviousness. Indeed, their KIT numbers (UM557) vary between 6 and 12, the latter being a value which can either not be achieved at all with conventional coating compositions or at best with significant difficulty. While the fat imperviousness of the amylose derivatives, as coatings on such porous substances as paper or cloth, is remarkable, they biodegrade relatively quickly when brought into contact with water. As coatings on paper-based wrappers for food having a high fat content but low water content, such as, for instance, biscuits or crackers, chocolate and other sweets, as well as for dry foods for humans and dry animal feed, the amylose derivatives have been found to be of exceptional quality. Coatings of amylose derivatives on otherwise porous base materials possess excellent qualities of imperviousness in respect of fat or fatty substances in environments where imperviousness against water vapors is of secondary importance, as is often the case with food wrappers.
The invention will hereafter be described in greater detail on the basis of preferred embodiments.
In the following examples, the basic material is a slightly yellow powder-like starch derived from peas and has:
a water content of about 11.5 percent by weight, it being understood that the water content is a function of storage conditions;
an amylose content of about 77.4%;
a protein content of about 0.9%; and
a median molecular weight between about 105 and about 2xc3x97107 g/mol.
As is well know and in the context of the invention herein described, amylose may be defined as a sol substituent of starch and consists of straight chain glucose molecules.
Having regard to the utilization of amylose derivatives as coatings for fat impervious laminates, those consisting of a starch derived with an alkylene oxide such as, for instance, propylene oxide have been found to be of particular advantage. In this connection, starches having a high proportion of amylose are preferred. An especially suitable derivative proving an enduring internal softening effect is hydroxypropylether starch in the range of about 0.05 to about 1.5 and preferably in the range between about 0.1 and 1.0.